cmd/compile: fix buglet in walk convert phase relating to convT64

The function runtime.convT64 accepts a single uint64 argument, but the
compiler's rules in the walk phase for determining whether is it ok to
pass a value of type T to a call to runtime.convT64 were slightly off.
In particular the test was allowing a type T with size less than eight
bytes but with more than one internal element (e.g. a struct). This
patch tightens up the rules somewhat to prevent this from happening.

Updates #40724.

Change-Id: I3b909267534db59429b0aa73a3d73333e1bd6432
Reviewed-on: https://go-review.googlesource.com/c/go/+/308069
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/walk/convert.go

@@ -14,6 +14,7 @@ import (
 	"cmd/compile/internal/ssagen"
 	"cmd/compile/internal/typecheck"
 	"cmd/compile/internal/types"
+	"cmd/internal/objabi"
 	"cmd/internal/sys"
 )
 
@@ -316,6 +317,14 @@ func convFuncName(from, to *types.Type) (fnname string, needsaddr bool) {
 		return false
 	}
 
+	// Helper to determine whether a given type (when passed to a
+	// function) will fit into a single integer register, assuming
+	// that the reg abi is in effect. This is somewhat ad-hoc, there
+	// may be a cleaner way to do this.
+	fitsInSingleIntReg := func(t *types.Type) bool {
+		return from.IsScalar() || types.IsDirectIface(from)
+	}
+
 	tkind := to.Tie()
 	switch from.Tie() {
 	case 'I':
@@ -332,7 +341,7 @@ func convFuncName(from, to *types.Type) (fnname string, needsaddr bool) {
 			return "convT32", false
 		case from.Size() == 8 && isFloatLike(from):
 			return "convT64F", false
-		case from.Size() == 8 && from.Align == types.Types[types.TUINT64].Align && !from.HasPointers():
+		case from.Size() == 8 && from.Align == types.Types[types.TUINT64].Align && !from.HasPointers() && (!objabi.Experiment.RegabiArgs || fitsInSingleIntReg(from)):
 			return "convT64", false
 		}
 		if sc := from.SoleComponent(); sc != nil {

test/abi/convT64_criteria.go

@@ -0,0 +1,25 @@
+// run
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"fmt"
+)
+
+type MyStruct struct {
+	F0 [0]float64
+	F1 byte
+	F2 int16
+	_  struct {
+		F0 uint32
+	}
+}
+
+func main() {
+	p0 := MyStruct{F0: [0]float64{}, F1: byte(27), F2: int16(9887)}
+	fmt.Println(p0)
+}

test/abi/convT64_criteria.out

@@ -0,0 +1 @@
+{[] 27 9887 {0}}